J-multiplied HSQC (MJ-HSQC): a new method for measuring 3J(HNHalpha) couplings in 15N-labeled proteins.

A new method for the measurement of homonuclear 3J(HNHalpha) coupling constants in 15N-labeled small proteins is described. The method is based on a modified sensitivity enhanced HSQC experiment, where the 3J(HNHalpha) couplings are multiplied in the f1-dimension. The J-multiplication of homonuclear 3J(HNHalpha) couplings is based on simultaneous incrementation of 15N chemical shift and homonuclear coupling evolution periods. The time increment for the homonuclear coupling evolution period is chosen to be a suitable multiple (2N x t1) of the corresponding increment for 15N-shift evolution. This results in the splitting of the HSQC correlation in the f1-dimension by 2N x 3J(HNHalpha). Because the pulse sequence has good sensitivity and water suppression properties, it is particularly useful for natural abundance samples.